While running in a liner string it is advantageous to circulate fluids through the liner and through its distal end. A toe circulation sub, also called a toe sub, may be installed at the distal end of the liner.
This fluid flow helps to keep any debris that is loose in the well from stopping the liner from progressing downhole. If the toe sub gets plugged then the flow is stopped and the liner may get stuck while running in. The toe sub also serves the purpose of allowing cleaning of any drilling mud from the openhole on the outside of the liner before the packers are set. A typical toe sub has a torturous path through it and, thus, it is prone to clogging. The closing of this toe sub is actuated by the use of a ball. This ball is dropped from the surface and pumped down to the toe sub. Once it contacts the seat in the toe sub it closes the flow path. This closed flow path creates a piston. The piston is pushed down by the applied pressure. Once the piston moves down the flow path is closed from the liner to the openhole. This is permanently closed and can't be re-opened. The ball is loose in the liner and can roll back down the liner and possibly get in the way of intervention tools.